The use of a support having coated thereon an AgX emulsion containing tabular grains having a large aspect ratio (diameter/thickness) in a photographic material is advantageous in the following points. For example, sharpness is improved by capability of reduction in the film thickness, a spectral sensitizing dye can be adsorbed in a large quantity by a great surface/volume ratio, a light absorptivity is improved, development processing is expedited by a great surface/volume ratio and granularity is improved by levelling of an image. Accordingly, tabular grains have hitherto been used so often in many photographic materials. However, when the tabular grain is produced by conventional methods, the following defects are involved. Non-tabular grains mingle together and the grain size distribution is broad. In other words, the grains obtained are polydispersed in view of the grain form (i.e., the grain shape) and the size distribution. As a result, if the grains are subjected to chemical sensitization or spectral sensitization, it fails to effect optimal chemical sensitization or spectral sensitization on all grains and thus, a multilayer effect is diminished.
In order to overcome this disadvantage, various investigations have been made from a technical viewpoint. The present inventors have made investigations on optimal conditions for three respective steps, namely, nucleation, ripening and growing steps constituting the production procedure of a tabular grain containing parallel twin planes. More specifically, the matters investigated are such that in the nucleation step, the twin plane formation probability is controlled not too high but not too low. In the ripening step, using the selective growth property of tabular grains at a low supersaturation degree, tabular grains are allowed to remain and other non-tabular grains vanish. And, in the growing step, by selecting the concentration or super-saturation degree of halogen ions (hereinafter referred to as "X.sup.- ") so as to achieve a selective growth property of a tabular grain and a diffusion rate-determining growth property at edge portions, the growth is advanced without broadening the size distribution. The following literatures describe thereon in detail and can be referred to.
With respect to the details of a tabular grain having a Cl.sup.- content of 50 mol % or more, U.S. Pat. Nos. 5,176,992, 5,061,617, 4,400,463, 5,185,239, 5,183,732, 5,178,998 and 5,178,997, JP-A-4-283742 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-4-161947 may be referred to, and with respect to grains having a high Br.sup.- content, JP-A-63-151618, JP-A-63-11928, JP-A-2-28638, JP-A-1-131541, JP-A-2-838, JP-A-2-298935 and JP-A-3-121445 may be referred to.
On the other hand, in the case of a tabular grain having {100} faces as main planes, if the grain is produced by conventional methods, similar problems are also caused. For the betterment thereof, the grain production formulation is parted into three steps, namely, nucleation, ripening and growing steps, and improved methods for respective steps have been proposed. JP-A-5-281640, JP-A-5-313273, U.S. Pat. Nos. 4,063,951, 4,386,156, 4,946,772, 5,264,337 and 5,275,930 and European Patent 0534395A1 describe thereon in detail and can be referred to.
As a result of these investigations, monodispersibility is outstandingly improved from the aspects of the grain form and the grain size distribution. However, a problem is still in need of overcoming, that is, as the thickness of a tabular grain is reduced more and more, the resulting grain size distribution is broadened. Also, a method for producing a tabular grain having a lower fog density and further excellent sensitivity and granularity has been demanded. To cope with this demand, an attempt has been proposed to improve properties of the grain by changing the dispersion medium at the time of grain formation. For example, in Kelly, Journal of Photographic Science, Vol. 6, 16-22 (1958), an AgBrI tabular grain is formed by adding an aqueous AgNO.sub.3 solution to an aqueous solution containing oxidation-processed gelatin oxidized under various conditions and X.sup.-. Sheppard or many other authors have written on the use of oxidation-processed gelatin oxidized by H.sub.2 O.sub.2 or the like. For the details thereon, British Patent 245,456, French Patent 768,015 and Gelatin in Photography-Monographs on the Theory of Photography from the Research Laboratory of the Eastman Kodak Co., No. 3, D. Van Nostrand Co., New York may be referred to. Also, it is confirmed that a methionine group is converted into methionine sulfoxide upon oxidation and Journal of Photographic Science, Vol. 16, 68-69 (1968) describes thereon.
Recently, a tabular grain having a thickness of 0.2 .mu.m or less has been produced in an oxidation-processed gelatin dispersion medium solution having a methionine content of less than 30 .mu.mol/g, as described, for example, in JP-A-62-157024. When a tabular grain is produced using the above-described gelatin, a tabular grain thinner than the grain obtained using non-oxidized gelatin is formed at all temperatures range of 76.degree. C. or less. However, as the grain thickness is reduced, the size distribution of produced grains is still broadened. European Patent 514742A discloses a method for overcoming the above-described defect by forming an AgBr tabular grain having {111} faces as main planes in the presence of oxidized gelatin described above and a polyalkylene compound. However, the tabular grain obtained has main planes in the irregularly distorted equilateral hexagonal form and also fails to have sensitivity, granularity and fog density on a satisfactory level.